The semantic of this ReferenceTypeis to relate event sources in a hierarchical, non-looping organization. This ReferenceTypeand any subtypes are intended to be used for discovery of Eventgeneration in a Server. They are not required to be present for a Serverto generate an Eventfrom its source (causing the Event) to its notifying Nodes. In particular, the root notifier of a Server, the Server Objectdefined in OPC 10000-5, is always capable of supplying all Eventsfrom a Serverand as such has implied HasEventSource Referencesto every event source in a Server.
The SourceNodeof this ReferenceTypeshall be an Object or Viewthat is a source of Event Subscriptions. A source of Event Subscriptionsis an Objector Viewthat has its “SubscribeToEvents” bit set within the EventNotifier Attribute. The SourceNodemay also be an ObjectTypewhen referencing an InstanceDeclarationwhere an instance of the ObjectTypecontaining the InstanceDeclarationgenerates events. Note the ObjectTypeis not considered a source of Event Subscriptions.
Starting from Node“A” and only following Referencesof the HasEventSource ReferenceTypeor of its subtypes it shall never be possible to return to “A”. But it is permitted that, following the References, there may be more than one path leading to another Node“B”.